British doctors help nearly 20,000 people a year to die, according to one of the UK's leading authorities on euthanasia. The claim, the first public attempt by a credible expert to put a figure on 'assisted dying' rates, will reignite the emotive debate over the practice.

Dr Hazel Biggs, director of medical law at the University of Kent and author of Euthanasia: Death with Dignity and the Law, calculates that at least 18,000 people a year are helped to die by doctors who are treating them for terminal illnesses.

Biggs, who has submitted evidence to the House of Lords select committee which is examining Lord Joffe's private member's bill on Assisted Dying for the Terminally Ill, makes the claim in an article submitted to the European Journal for Health Law .

Her figures will place renewed focus on the doctor-patient relationship, which pro-euthanasia campaigners want changed so that medical staff can help conscious, terminally ill patients in pain to shorten their lives.

Biggs's figures are based on data from countries such as the Netherlands and Australia, which have published research into assisted dying rates, as well as evidence taken from British doctors.

'If you extrapolate from countries that have published data, you're looking at quite a large number of patients who may have had their end hastened, not necessarily with their consent,' she said.

'What this says to me is that we know these practices are going on, but they are completely unregulated. We don't know how many people are volunteers or non-volunteers, and maybe because of that the law ought to be changed so that people can give voluntary consent, which will give them more protection.'

An ageing population has meant that an increasing number of doctors are taking private decisions to aid the early demise of terminally ill patients, usually by increasing drug doses.

Deborah Annetts, chief executive of the Voluntary Euthanasia Society, said there was an urgent need to clarify regulations governing assisted dying: 'We need to shine a spotlight on this. The medical profession doesn't want the public to realise they are making these decisions. It shows the need to make the patient the decision-maker. When it's left to the doctor, there is always the risk of abuse.'

Pro-euthansia groups point out that in Britain the maximum sentence for helping someone to commit suicide is 14 years in prison. 'With the exception of Ireland, no other country in Europe behaves like that,' Annetts said.

Opinion polls show overwhelming public support for law changes that would make it easier for terminally ill patients in pain to request medical help to shorten their lives. In successive surveys, about 80 per cent of people back the move. A survey by the society this month found that 47 per cent of people said they were prepared to help a loved one to die, even if it meant breaking the law.

But a spokeswoman for the ProLife party said: 'Surely the response of a compassionate society is to alleviate the pain, to love and comfort the patient, and to try and restore a sense of self-worth until death comes naturally.'

Politicians have repeatedly deflected moves to change the law on euthanasia, believing it is unlikely to be a vote-winner. But Joffe's bill might find its way through the Lords committee stage and into the Commons, which would alarm religious groups.

In a joint submission to the select committee, Church of England and Roman Catholic bishops said: 'It is deeply misguided to propose a law by which it would be legal for terminally ill people to be killed or assisted in suicide by those caring for them, even if there are safeguards to ensure that only the terminally ill would qualify.'